1. Field of the Invention
The invention relates generally to the field of the evaluation of wellbore casing. More specifically, the present invention relates to a method and apparatus to provide for the analysis of the bond that secures casing within a wellbore. Yet even more specifically, the present invention relates to a method and apparatus that enables non-destructive testing of the bond securing casing within a wellbore where the testing includes the production and transmitting of multiple waveforms including compressional waves, shear waves, Lamb waves, Rayleigh waves, and combinations thereof, in addition to the receiving and recording of the waveforms within the casing.
2. Description of Related Art
Hydrocarbon producing wellbores typically comprise casing 8 set within the wellbore 5, where the casing 8 is bonded to the wellbore by adding cement 9 within the annulus formed between the outer diameter of the casing 8 and the inner diameter of the wellbore 5. The cement bond not only adheres the casing 8 within the wellbore 5, but also serves to isolate adjacent zones (Z1 and Z2) within the formation 18 from one another. Isolating adjacent zones can be important when one of the zones contains oil or gas and the other zone includes a non-hydrocarbon fluid such as water. Should the cement 9 surrounding the casing 8 be defective and fail to provide isolation of the adjacent zones, water or other undesirable fluid can migrate into the hydrocarbon-producing zone thus diluting or contaminating the hydrocarbons within the producing zone.
To detect possible defective cement bonds, downhole tools 14 have been developed for analyzing the integrity of the cement 9 bonding the casing 8 to the wellbore 5. These downhole tools 14 are lowered into the wellbore 5 by wireline 10 in combination with a pulley 12 and typically include transducers 16 disposed on their outer surface formed to be acoustically coupled to the fluid in the borehole. These transducers 16 are generally capable of emitting acoustic waves into the casing 8 and recording the amplitude of the acoustic waves as they travel, or propagate, across the surface of the casing 8. Characteristics of the cement bond, such as its efficacy and integrity, can be determined by analyzing the attenuation of the acoustic wave.
Typically the transducers 16 are piezoelectric devices having a piezoelectric crystal that converts electrical energy into mechanical vibrations or oscillations that can be transmitted to the casing 8 thereby forming acoustic waves in the casing 8. To operate properly however, piezoelectric devices must be coupled with the casing 8. Typically coupling between the piezoelectric devices and the casing 8 requires the presence of a coupling medium between the device and the wall of the casing 8. Coupling mediums include liquids that are typically found in wellbores. When coupling mediums are present between the piezoelectric device and the casing 8 they can communicate the mechanical vibrations from the piezoelectric device to the casing 8. Yet, lower density fluids such as gas or air and high viscosity fluids such as some drilling muds cannot provide adequate coupling between a piezoelectric device and the casing 8. Furthermore, the presence of sludge, scale, or other like matter on the inner circumference of the casing 8 can detrimentally affect the efficacy of a bond log with a piezoelectric device. Thus for piezoelectric devices to provide meaningful bond log results, they must be allowed to cleanly contact the inner surface of the casing 8 or be employed in wellbores, or wellbore zones, having liquid within the casing 8.
Another drawback faced when employing piezoelectric devices for use in bond logging operations involves the limitation of variant waveforms produced by these devices. Fluids required to couple the wave from the transducer to the casing with only effectively conduct compressional waves, thus limiting the wave types that can be induced in the casing, although many different types of acoustical waveforms are available that could be used in evaluating casing, casing bonds, and possibly even conditions in the formation 18.
Currently devices do exist that can detect flaws or failures within a wellbore casing, such as scaling, pitting, or other potentially weak spots within the casing. These devices create a magnetic field that permeates the casing, such that an inconsistency of material within the casing, such as potential weak spots, can be identified. Application of these devices is limited to conducting an evaluation of only the wellbore casing itself.
Therefore, there exists a need for the ability to conduct bond logging operations without the presence of a needed couplant. Furthermore, a need exists for a bond logging device capable of emitting numerous types of waveforms.